Extended Forecast
Valid: Sat 15 Mar 2008 06:00 to Sun 16 Mar 2008 06:00 UTC
Issued: Thu 13 Mar 2008 18:44
Forecaster: TUSCHY

SYNOPSIS:

The depression southwest of Ireland continues its eastward drift and gets through to the western part of the English Channel until midnight. This feature results in the first far northward extending elevated mixed layer of this year, overspreading Spain, the western Mediterranean and most parts of south / central France although this plume fans out during the end of the forecast period. The standard deviation maps hint on uncertainties, which can't be neglected, mainly how far south and with which exact tilt this trough will cross France during the evening and night hours but overall, model pool and run-to-run consistency are stable enough so that confidence is growing that a more active day unfolds for most parts of France.


... Bay of Biscay and France ...

We won't get into too many details that far out as models do not yet agree very well among each other. Strong upper-level streak on the upstream side of the depression curves around the base and stays the dominant one. This results in a flattening and a more zonal orientation of the depression. In addition, latest model outputs hint on numerous re- / new developments of the main surface low center during the forecast period, resulting in a quite diffuse LL wind field ( e.g. speed max at 850hPa stalls over the S-Bay of Biscay for quite a long time ). The main focus for thunderstorm initiation will be an upper-level low pressure channel and attendant cold front, crossing France from west to east during the daytime hours. Airmass ahead of this front gets quite unstable as dewpoints reach 10°C, and strong LL moisture advection helps to bring this moisture well towards the north. In combination with steep mid-level lapse rates, up to 800 J/kg nearly uncapped SBCAPE can be anticipated over central and southwest France.
Shear at all levels gets stronger towards the south and hence the main level-1 area was confined to those areas with the risk for an isolated large hail, severe wind gust and tornado report. We also expaned the level area well towards the northeast, over central France, as maximized instability release and up to 200 m^2/s^2 SRH 1 should be enough for an isolated tornado threat and large hail report along the eastward moving MCS.
Rapidly easing instability and a stabilizing boundary layer should help to weaken the line although the strong forcing could support a continuing elevated thunderstorm threat as far east as SW Germany.

There are indications that a new speed max at mid-levels could enhance the severe wind gust risk over SE-France / W-Switzerland and extreme SW Germany during the evening hours, but we want to see a confirmation in the next model runs before we reflect this in the risk level ( e.g. an eastward expansion of the level-1 area ).

During the night hours, the mid-level cold pool finally overspreads W - France and a few more thunderstorms will evolve. LL helicity is enhanced and this area has to be monitored for an augmented tornado risk and an upgrade to a level-1.